we need full details.
lights, light height, medium your using, are you feeding? if your medium charged with nutrients?, what are you currently giving it? etc..
here are just my assumptions that could be the reasons.
1. over watering. try not to over water it
2. medium your using might be high/no charge. if there is no charge i'd start feeding imo since it looks old enough to handle some feeding.
3. might need cal mag i run rain water so i don't use cal mag but idk what you run. even if i ran tap water i wouldn't need cal mag but need to let it breath for 24 hours to remove most of the chlorine.
4. your soil might be charged and you might be feeding as well which is an issue. if running coco i can't help you with that as i have no experience with coco.
5. PH is off balance. measure your PH both going in and via run off.